1. Field of the Invention
The present invention relates to a laminated electronic part, a method for the production thereof, and a dielectric ceramic composition and, more particularly, to a laminated ceramic capacitor having a capacitance as low as from approximately 0.01 pF to 30 pF, suitable for use in a high frequency region of, for example, from several hundreds MHz to several GHz, a method for the production thereof, and a dielectric ceramic composition suitable for use in forming a dielectric ceramic layer of a such laminated electronic part.
2. Description of the Related Art
A variety of substances is known as a material for a dielectric ceramic layer of a laminated ceramic capacitor having a low capacitance for use in a high frequency region. One of such substances is, for example, a dielectric ceramic composition of a (Mg.sub.a Zn.sub.(1-a)).sub.x SiO.sub.x+2 (hereinafter sometimes referred to as "MZS")--Al.sub.2 O.sub.3 --SrTiO.sub.3 type.
As silica materials for use as part of the MZS, there has generally been employed a crystalline silica material. The major component of the crystalline silica material is SiO.sub.2 and this component is very hard. Further, silica material commercially available is one of a crystalline type (.alpha.-quartz) and so large in particle size (an average particle size being 3 .mu.m or larger and the maximum size being 8 .mu.m or larger) and so wide in a particle distribution. Moreover, such silica material is so low in a fracture toughness value that it is likely to become an isotropic in shape (an angular shape) when it is finely divided into fine particles by grinding.
In order to obtain finely divided silica powder, there has hitherto been employed a method which involves classifying a supernatant of a suspension of pulverized silica powders. This method, however, can provide fine silica powders having particle sizes as small as approximately 1.5 .mu.m and yet having angular shapes.
Therefore, the MZS powders prepared from such silica powders having such large particle sizes and angular shapes have particles shapes of such silica powders as raw materials still left therein, resulting in angular shapes having coarse or rough surfaces.
When a green sheet is prepared from such MZS of an angular shape, the surface of the resulting green sheet may become coarse or rough, thereby impairing uniformity in the thickness of a dielectric ceramic layer and worsening a distribution of capacitance of laminated ceramic capacitors.
Further, if the green sheet becomes coarse or rough on its surface, an internal electrode to be formed thereon may be made irregular and uneven on its surface, thereby elevating a surface resistance of the internal electrode and impairing frequency properties of factor Q, particularly in a high frequency region.
Moreover, the such green sheet may become narrow in an optimum range of binder amounts, thereby making the binder amount likely to become excessive or too small. If the binder amount would become too large, on the one hand, mold flashing may be occurred to a great extent upon cutting into chips and such flashes cannot be thoroughly removed even if they are to be processed with a barrel. If the binder amount would be too small, on the other, a sheet-binding strength may be decreased, thereby causing the laminated layers to deviate from each other or air to be mixed therein. This may cause a decrease in reliability.
Moreover, the green sheet prepared from the such MZS has a high shrinkage initiation temperature so that the resulting laminated ceramic capacitors may be delaminated readily upon calcination.
A copending patent application is directed to a dielectric ceramic composition of a MZS--Al.sub.2 O.sub.3 --SrTiO.sub.3 type as represented by the following general formula: EQU X(Mg.sub.a Zn.sub.(1-a)).sub.x SiO.sub.x+2 --YAl.sub.2 O.sub.3 --ZSrTiO.sub.3
(where symbol a is defined by: 0.1.ltoreq.a.ltoreq.0.8; and symbol x is defined by: 0.67.ltoreq.x.ltoreq.1.5); PA1 (where symbol a is defined by: 0.1.ltoreq.a.ltoreq.0.8; and symbol x is defined by: 0.67.ltoreq.x.ltoreq.1.5);
in which a mole percent ratio of (Mg.sub.a Zn.sub.(1-a)).sub.x SiO.sub.x+2 to Al.sub.2 O.sub.3 to SrTiO.sub.3 is located in a region enclosed by a polygon having apexes at points A, B, C and D in a three-component composition map as defined as follows:
______________________________________ A (94.9, 0.1, 5.0) B (85.0, 10.0, 5.0) C (65.0, 10.0, 25.0) D (65.0, 0.1, 34.9). ______________________________________
This dielectric ceramic composition has superior properties that it can be sintered at temperature of 1,100.degree. C. or lower, a dielectric constant .epsilon..sub.r is as low as 15 or less and no delamination may be caused to occur even when pure Pd is used as material for an internal electrode.
The dielectric ceramic compositions involved in the copending patent application, however, suffers from the difficulty that a somewhat large amount of leak current may be caused when it is used for a dielectric ceramic layer of laminated ceramic capacitors.